For the last two decades, resin materials known as ionomers have been used extensively as cover stock materials for golf balls. It has been found that such covers provide acceptable in-play characteristics to the balls, such as initial velocity, spin rate and feel.
These resins are well known to those of ordinary skill in the golf ball art and are commercially available under a variety of trade names such as SURLYN.RTM. (DuPont) and IOTEK.RTM. (Exxon). Presently, there are more than 50 grades of ionomers commercially available having a wide range of properties which vary according to a variety of parameters such as the type and amount of metal cations, molecular weight and composition of the base resin (e.g., the relative content of ethylene and methacrylic and/or acrylic acid groups).
Generally speaking, commercial ionomers consist of a copolymer of a mono-olefin, e.g., an alkene, with at least one other type of comonomer selected from the group consisting of unsaturated mono- or di-carboxylic acids having 3 to 12 carbon atoms and esters thereof (the polymer contains 1 to 50% by weight of the unsaturated mono- or di-carboxylic acid and/or ester thereof) with from about 10% to about 90% of the carboxylic acid groups neutralized by a metal ion, e.g., see U.S. Pat. No. 3,264,272. More particularly, such acid-containing ethylene copolymer ionomers include E/X/Y copolymers where E is ethylene, X is a softening comonomer such as acrylate or methacrylate present in an amount of from 0 wt. % to about 50 wt. % of the polymer, and Y is acrylic or methacrylic acid present in an amount from about 5 wt. % to about 35 wt. % of the polymer, wherein the acid moiety is neutralized from about 1% to about 90% to form an ionomer by a cation such as lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc or aluminum, or a combination of such cations.
"High acid ionomers", as used herein and as known to those of ordinary skill in the art, are those ionomer resins wherein Y is acrylic or methacrylic acid units present from 16.5 wt. % to about 35 wt. % in the polymer. Generally, a high acid ionomer will have a flexural modulus from about 50,000 psi to about 125,000 psi. In the vernacular of the golf ball art, high acid ionomers are sometimes referred to as "hard" ionomers.
"Very low modulus ionomers", or "VLMI", as used herein and as known to those of ordinary skill in the art, are those ionomer resins further comprising a softening comonomer X, commonly a (meth)acrylate ester, present from about 10 wt. % to about 50 wt. % in the polymer. Typical softening comonomers include n-butyl acrylate, iso-butyl acrylate, n-butyl methacrylate, methyl acrylate and methyl methacrylate. Generally, a VLMI will have a flexural modulus from about 2,000 psi to about 10,000 psi. VLMI are sometimes referred to as "soft" ionomers.
"Standard ionomers", as that term is used herein and as known to those of ordinary skill in the art, are those ionomer resins generally without a softening comonomer X and wherein Y is acrylic or methacrylic acid units present from about 1 wt. % to &lt;16.5 wt. % in the polymer. Generally, a standard ionomer will have a flexural modulus from about 20,000 psi to about 80,000 psi. Standard ionomers are also sometimes referred to as "hard" ionomers in the art to distinguish them from VLMI.
Cover compositions formed from blends of standard ionomers typically provide balls with acceptable resilience and distance characteristics.
A number of patents are directed towards improved ionomer blends comprising high acid ionomers. U.S. Pat. No. 5,298,571 to Statz et al., for example, discloses blends of ionomers, each ionomer comprising between 16 wt. % and 25 wt. % acid groups. Such blends are further limited in that the coefficient of restitution of a golf ball comprising a cover formed from these blends must be greater than 0.7. The coefficient of restitution, or "COR", is a measure of resilience of a golf ball well known to those in the golf ball arts. Its value increases as a golf ball becomes more resilient, i.e., as the amount of energy lost when a golf ball rebounds from a collision with a steel plate decreases. Furthermore, the blends disclosed by the subject reference are neutralized with only lithium, zinc and, optionally, sodium, all present within prescribed ratios. The Statz et al. reference, however, does not disclose any blends comprising high acid ionomer(s) and a VLMI.
Golf ball covers, such as those described above comprising standard ionomer(s), high acid ionomer(s) and/or blends thereof, provide golf balls with high initial velocity. However, golf balls formed with such covers are known to have a low spin rate, a characteristic which is looked upon unfavorably by skilled golfers. Therefore, other patents and patent publications are directed toward improving the spin rate and other desirable cover properties, such as click and feel, by forming cover compositions from ionomer blends comprising a VLMI. For example, U.S. Pat. No. 4,884,814 to Sullivan discloses golf ball covers comprising blends of a high flexural modulus (hard) ionomer and a low flexural modulus (soft) ionomer. The hard ionomers are copolymers of an olefin and an unsaturated monocarboxylic acid, neutralized with sodium or zinc, and have a flexural modulus of about 30,000 to 55,000 psi and a Shore D hardness of about 60 to 66; SURLYN.RTM. 9910, 8528, 8940 and 9650 are exemplified. None of the exemplified hard ionomers appear to have an acid content exceeding 15 wt. %, thus, they are standard ionomers. The soft ionomers are terpolymers of an olefin, an unsaturated monocarboxylic acid, and an acrylate ester, i.e., VLMIs. These terpolymers are neutralized with zinc or sodium and have a flexural modulus of about 3,000 to 7,000 psi and a Shore D hardness of about 25 to 40; SURLYN.RTM. AD-8269 and AD-8265 are exemplified. The resulting cover is disclosed as being softer than a hard-ionomer covered ball such that adequate spin can be imparted to the ball by a skilled golfer. However, this reference does not disclose a blend of a VLMI and a high acid ionomer.
U.S. Pat. No. 5,120,791 to Sullivan, closely related to the previous reference, discloses golf ball cover compositions containing blends of at least one hard ionomeric resin, preferably an acrylic acid ionomer, and at least one acrylic acid based soft ionomer. Hard ionomers neutralized with sodium, zinc, magnesium or lithium and having a Shore D hardness of 55 to 66 are disclosed; SURLYN.RTM. 8940 and 9910, each containing about 15 wt. % of methacrylic acid, and, preferably, IOTEK.RTM. 4000 and 8000, containing about 16 and 11 wt. % acrylic acid, respectively, are disclosed. The disclosed soft ionomers are sodium or zinc neutralized terpolymers of an olefin, acrylic acid, and an unsaturated acrylate ester monomer, i.e., VLMIs, and have a flexural modulus of about 2,000 to 10,000 psi and a Shore D hardness of about 20 to 50; IOTEK.RTM. 7520, an ethylene/methyl acrylate/acrylic acid terpolymer partially neutralized with zinc, is exemplified. However, this reference does not disclose a blend of a VLMI and a high acid ionomer.
UK Patent Application GB 2,278,609 A discloses a three-piece golf ball with a cover layer formed from blends of a high flexural modulus (hard) ionomer and a low flexural modulus (soft) ionomer. The hard ionomers are disclosed to be copolymers of an olefin and an unsaturated monocarboxylic acid, neutralized with sodium, zinc, magnesium or lithium, having a flexural modulus of about 15,000 to 70,000 psi and a Shore D hardness of at least 50; SURLYN.RTM. 8940 and 9910, each containing about 15 wt. % of methacrylic acid, and IOTEK.RTM. 4000 and 8000, containing about 16 and 11 wt. % acrylic acid, respectively, are exemplified. Thus, these hard ionomers are standard ionomers and not high acid ionomers. The soft ionomers are disclosed to be terpolymers of an olefin, an unsaturated monocarboxylic acid, and an acrylate ester, i.e., VLMIs. These terpolymers are neutralized with zinc or sodium, and have a flexural modulus of about 1,000 to 10,000 psi and a Shore D hardness of about 25 to 40; IOTEK.RTM. 7520 and 7510, each consisting of an ethylene/methyl acrylate/acrylic acid terpolymer partially neutralized with zinc, are exemplified. However, this reference does not disclose a blend of a VLMI and a high acid ionomer.
While the above-described cover blends comprising VLMIs improve playability, i.e., increase spin rate and improve click and feel, it is recognized that other desirable golf ball properties, particularly the resilience (e.g., COR) and distance properties, are negatively impacted by the inclusion of the VLMI.
PCT Patent Publication No. WO 95/00212 discloses an ionomer cover blend comprising a VLMI and another ionomer, i.e., an ethylene copolymer with acrylic or methacrylic acid and having an acid level of from 14 to 22 wt. %, which may be a high acid ionomer. These cover compositions must contain relatively large amounts, at least 15 wt. % and preferably at least 45 wt. %, of a VLMI component which is characterized as soft, i.e., with a flexural modulus from 1,000 to 10,000 psi, blended with the other ionomer component characterized as hard, i.e., with a flexural modulus of 50,000 to 110,000 psi. However, the durability of these disclosed cover compositions is poor.
U.S. Pat. No. 5,415,937 to Cadorniga et al. discloses a cover blend comprising a high acid ionomer and a VLMI which is said to improve the feel and playability of the ball while also retaining the distance and resilience of prior art balls containing no VLMI component. In particular, a golf ball cover blend of 80 to 50% of a high stiffness ionomer and 20 to 50% of a VLMI is disclosed. This reference defines high stiffness ionomers as a copolymer of 75 to 80% of an olefin and 25 to 20% of an alpha, beta ethylenically unsaturated carboxylic acid, having 10 to 90% of the acid groups neutralized with sodium, lithium, zinc, or magnesium. The high stiffness ionomers preferably have a flexural modulus of 60,000 to 120,000 psi and a Shore D hardness of at least 70. The VLMI are defined as terpolymers, consisting of 67 to 70 wt. % of ethylene, 23 to 20 wt. % of n- or iso-butyl-acrylate, and 10 wt. % of methacrylic acid, where 10 to 90% of the acid groups are neutralized by sodium, zinc, or lithium. The VLMI preferably have a flexural modulus of 2,000 to 8,000 psi and a Shore D hardness of 20 to 50. Golf balls having covers incorporating the disclosed blends have a slightly improved COR and initial velocity with spin rates that range from slightly better than prior art blends to significantly lower, depending upon the particular blend and the club used in the test, i.e., a driver, a 5-iron or a pitching wedge. However, a very high amount of VLMI, at least 20%, still is present in all of the Cadorniga et al. ionomer blends.
While some of the high acid ionomer and VLMI cover blends of Cadorniga et al. provide an acceptable combination of initial velocity and playability, golf balls formed from such covers are known to suffer from poor durability, particularly as demonstrated by their tendency to suffer cover cracks at a low numbers of hits.
Therefore, despite the above-described advantages of golf balls with ionomer blend covers, there exists a need for golf balls comprising high acid ionomers having superior cover durability when compared to existing golf balls with high acid/standard ionomer and with high acid/VLMI cover blends. Furthermore, there exists a need for golf balls which have improved initial velocity and COR when compared to existing golf balls comprising high acid/VLMI cover blends.